1. Field of the Invention
This invention relates to a process for forming a deposited film, particularly a process for forming a deposited film composed mainly of Al which can be preferably applied to electrodes or wiring of a semiconductor integrated circuit device, etc. and to a process for preparing a semiconductor device.
2. Related Background Art
In the prior art, in electronic devices or integrated circuits by use of semiconductors, for electrodes and wiring, aluminum (Al) or a metal containing aluminum as main component such as Al--Si and the like has been primarily used. Al has many advantages such as that it is inexpensive and high in electroconductivity, that it can be also internally chemically protected because a dense oxidized film can be formed on the surface, and that it has good adhesion to Si, etc.
As the method for forming a film for electrodes and wiring of Al or Al--Si as mentioned above, there has been used in the prior art the sputtering method such as magnetron sputtering, etc.
However, selective deposition and selective growth can not be realized by the sputtering method which has been used in the prior art. The sputtering is the physical deposition method based on flying of sputtered particles in a vacuum. The thickness at the stepped portion or the insulating film side wall becomes extremely thin, leading to wire breaking in an extreme case. Nonuniformity of film thickness or wire breaking has the drawback that reliability of LSI is markedly lowered.
On the other hand, since the integration degree of the integrated circuit such as LSI, etc. is increased, and fine formation of wiring or multi-layer wiring has been particularly required in recent years, there is an increasing great demand not found up to date for Al or Al--Si wiring of the prior art. With finer dimensional formation by increased integration degree, the surface of LSI, etc. is subject to excessive unevenness due to oxidation, diffusion, thin film deposition, and etching, etc. For example, electrodes or wiring metal must be deposited on the surface with a stepped difference, or deposited in a via-hole which is fine in diameter and deep. In 4 Mbit or 16 Mbit DRAM (dynamic RAM), etc., the aspect ratio (via-hole depth/via-hole diameter) of via-hole in which a metal composed mainly of Al such as Al--Si, etc. is to be deposited, is 1.0 or more, and the viahole diameter itself also becomes 1 Mm or less. Therefore, even for a via-hole with large aspect ratio, the technique which can deposit an Al--Si compound is required.
Particularly, for performing sure electrical connection to the device under insulating film such as SiO.sub.2, etc., rather than film formation, Al--Si is required to be deposited so as to embed only the viahole of the device. In such case, a method of depositing an Al alloy only on Si or metal surface and not depositing it on an insulating film such as SiO.sub.2, etc. is required.
As the improved sputtering method, there has been developed the bias sputtering method in which a bias voltage is applied on a substrate and deposition is performed so as to embed Al or an Al--Si compound only in the via-hole by utilizing the sputter etching action and the deposition action on the substrate surface. However, since the bias voltage of some 100 V or higher is applied on the substrate, deleterious influence on the device occurs because of charged particles damages such as change in threshold of MOSFET, etc. Also, because of presence of both etching action and deposition action, there is the problem that the deposition speed cannot be essentially improved.
In order to-solve the problems as described above, various types of CVD (Chemical Vapor Deposition) methods have been proposed. In these methods, chemical reaction of the starting gas in some form is utilized. In plasma CVD or optical CVD, decomposition of the starting gas occurs in gas phase, and the active species formed there further react on the substrate to give rise to film formation. In these CVD methods, surface coverage on unevenness on the substrate surface is good. However, carbon atoms contained in the starting gas molecule are incorporated into the film. Also, particularly in plasma CVD, the problem remained that there was damage by charged particles (so called plasma damage) as in the case of the sputtering method.
The thermal CVD method, in which the film grows through the surface reaction primarily on the substrate surface, is good in surface coverage on unevenness such as stepped portion of the surface, etc. Also, it can be expected that deposition within via-hole will readily occur. Further, wire breaking at the stepped portion can be avoided. Furthermore, there are no damages caused by charged particles which have been observed in the case of the plasma CVD or the sputtering method. The methods of such a kind include, for example, the method seen in Journal of Electrochemical Society, Vol. 131, p. 2175 (1984). In this method triisobutyl aluminum (i-C.sub.4 H.sub.9).sub.3 Al (TIBA) is used as organic aluminum gas and an Al film is formed at a film formation temperature of 260.degree. C. and a reaction tube pressure of 0.5 Torr. The substrate temperature is then maintained at about 450.degree. C. and SiH.sub.4 is introduced to diffuse Si in the Al film, thus obtaining an Al--Si film.
When TIBA is to be used, a continuous film can not be obtained unless pretreatment is effected in which prior to film formation, TiCl.sub.4 is flowed to activate the substrate surface to form nuclei. Furthermore, in case where TIBA is used, there is a problem that surface flatness is not sufficient even when TiCl.sub.4 is used. By this method it is not possible to effect selective growth such as Al--Si deposition only in via-holes.
As another method, there is the method described in Japanese Journal of Applied Physics, Vol. 27, No. 11, p. L2134 (1988). In this method, TIBA and Si.sub.2 H.sub.6 diluted with Ar gas are supplied, and the gases are heated before TIBA reaches the substrate. By this method, an Al--Si film of a low resistivity can be epitaxially grown on a Si (100) wafer. Although the film obtained by this method has good high quality, there are problems involving difficulty in process control due to the necessity of heating the gases and a complicated apparatus for effecting this method.
Japanese Laid-Open Patent Application No. 63-33569 describes a method of forming a film by using no TiCl.sub.4, but using in place thereof organic aluminum and heating it in the vicinity of the substrate.
According to this method, Al can be deposited selectively only on the metal or semiconductor surface from which the naturally oxidized film has been removed.
In this case, it is clearly stated that the step of removing the naturally oxidized film on the substrate surface is necessary before introduction of TIBA. Also, it is described that, since TIBA can be used alone, no carrier gas is required, but Ar gas may be also used as the carrier gas. However, the reaction of TIBA with another gas (e.g. H.sub.2) is not contemplated at all, and there is no description of using H.sub.2 as the carrier gas. Also, in addition to TIBA, trimethyl aluminum (TMA) and triethyl aluminum (TEA) are mentioned, but there is no specific description of other organic metals. This is because, since the chemical properties of organic metals generally vary greatly if the organic substituent attached to the metal element varies little, it is necessary to investigate individually by detailed experimentation to determine what organic metal should be used.
In the CVD method as described above, not only is there is the inconvenience that the naturally oxidized film must be removed, but also there is the drawback that no surface smoothness can be obtained. Also, there is the restriction that heating of the gas is necessary, and yet heating must be done in the vicinity of the substrate. Moreover, it must also be experimentally determined at what proximity to the substrate heating must be done, whereby there is also the problem that the place for setting the heater cannot be necessarily freely chosen.
In the pre-text of the 2nd Symposium of Electrochemical Society, Branch of Japan (Jul. 7, 1989), on page 75, there is a description of film formation of Al according to the double wall CVD method. In this method, TIBA is used and the device is designed so that the gas temperature of TBA can be made higher than the substrate temperature. This method may be also regarded as a modification of the above-mentioned Japanese Laid-open Patent Application No. 63-33569. Also in this method, Al can be selectively grown only on a metal or semiconductor, but not only the difference between the gas temperature and the substrate surface temperature can be controlled with difficulty, but also there is the drawback that the bomb and the pipeline must be heated. Moreover, according to this method, there are involved such problems that no uniform continuous film can be formed, that flatness of the film is poor, etc., unless the film is made thick to some extent.
As set forth above, the prior art methods have the problems that it is difficult to obtain a flat Al--Si film of a low resistivity and good quality, that there is considerable limitation in apparatus and in film formation.
As described above, prior art methods cannot necessarily effect well selective growth of Al--Si, and even if possible, there is a problem with respect to flatness, resistance, purity, etc. of the Al film formed. Also, there has been involved the problem that the film formation method is complicated and can be controlled with difficulty.
Furthermore, according to the prior art as described above high throughput sufficient to get a final commercial success has not been achieved.